A. Field of the Invention
The present invention relates to a method of manufacturing a perpendicular magnetic recording medium mounted on magnetic recording devices (hard disk drives (HDDs) used for external memory devices for computers, AV devices and the like). In particular, the invention relates to a method of manufacturing a discrete track medium type perpendicular magnetic medium in which a concentric configuration of protrusions and recesses is formed on the surface of a magnetic recording layer of the perpendicular magnetic recording medium along the track direction, and magnetic interaction (interference) between the tracks is reduced, thereby enhancing the recording density.
B. Description of the Related Art
A recording density of hard disk drives (HDDs) is rapidly increasing recently. In order to further advance enhancement of the recording density of perpendicular magnetic recording media, it is necessary to promote magnetic separation between crystalline particles composing a magnetic recording layer and to decrease a magnetization inversion unit. It is important for this purpose to ensure physical separation of the magnetization inversion units and suppress the magnetic interaction between adjacent recording units.
In order to cope with this problem, patterned medium type magnetic recording media have been proposed including a discrete track type medium (also referred to simply as a DTM in the following description) and a bit patterned type medium (also referred to simply as a BPM). The patterned media type magnetic recording media can suppress blur to adjacent tracks in a writing process.
In a DTM type magnetic recording medium, a concentric pattern of protrusions and recesses as mentioned above is formed on a perpendicular magnetic recording layer deposited on a nonmagnetic substrate as a continuous film. In a BPM type magnetic recording medium, a desired pattern of protrusions is formed to provide an uneven structure. Recording tracks formed in the protruding parts in both types of patterned medium type magnetic recording media are separated physically and magnetically with adjacent recessed parts and are independent of one another, to clearly isolate the magnetization inversion units.
The pattern of protrusions and recesses is formed using a carbon film or the like for a mask material on the surface of a magnetic recording medium by removing the unmasked parts by a dry etching method such as an ion beam etching with inert gas or reactive ion etching to create recessed parts.
Of the patterned media type magnetic recording media, the DTM type magnetic recording medium is effectively manufactured by providing on a perpendicular magnetic recording layer a concentric pattern of protrusions and recesses along a track direction. Various related prior technologies have already been proposed.
The technology disclosed in Japanese Unexamined Patent Application Publication No. H04-310621 and S56-119934, provides void regions in the recording magnetic layer between tracks. These documents also disclose a magnetic recording layer formed in a spiral shape to reduce side crosstalk on reproduction and enhance track density.
Japanese Patent No. 2513746 discloses a perpendicular magnetic recording medium in which nonmagnetic guard bands for isolation between recording tracks formed in the perpendicular magnetic recording layer are formed on the soft magnetic backing layer.
Japanese Unexamined Patent Application Publication No. 2003-016622 and 2006-012285 disclose a magnetic recording medium having a recording layer separated into a multiple of recording elements in a prescribed pattern of protrusions and recesses to surely hinder erroneous writing on adjacent tracks.
Japanese Unexamined Patent Application Publication No. 2002-288813 discloses a magnetic recording medium with a smooth surface comprising a plurality of magnetic sections and separation sections that surround the magnetic sections and have a magnetic property modified by ion implantation.
Japanese Unexamined Patent Application Publication No. 2002-359138 discloses a method in which a ferromagnetic layer is selectively masked with a resist and selectively modified to create nonmagnetic parts by exposing to active reactive plasma ions containing halogen to separate the ferromagnetic parts. Similar description is made in Japanese Unexamined Patent Application Publication No. 2008-065944.
The etching process is favorably executed in process conditions that completely separate the magnetic recording layer and with a minimum etching amount to preserve head flying performance and environment resistance. More specifically, a process for magnetic separation between tracks has been proposed in which the top surface layer of a magnetic recording layer is removed and the resulted recessed parts are exposed to reactive ion plasma containing halogen such as fluorine in order to obtain nonmagnetic parts by modifying the magnetic properties through chemical reaction.
The proposal, although it does not mention a specific mechanism, utilizes the fact that a magnetic recording layer with a granular structure employed in an ordinary perpendicular magnetic recording medium with SiO2 filled between magnetic particles can be readily modified to become a nonmagnetic layer by modifying magnetic properties thereof with reactive ion plasma containing halogen. Since the technology actively utilizes the reactive ion plasma containing halogen gas, the magnetic recording layer suffers from development of corrosion in some extent.
Japanese Unexamined Patent Application Publication No. 2009-076146 discloses a method for avoiding deterioration of long term stability of recording data due to corrosion by eliminating, through irradiation of inert gas ions, the surface layer of the magnetic recording layer corroded in the process using the etching gas containing halogen.
Japanese Unexamined Patent Application Publication No. 2009-026435 discloses formation of a magnetic recording pattern with a magnetically separated magnetic layer by exposing parts of the magnetic layer surface to reactive plasma of oxygen or halogen to selectively transform the magnetic layer to amorphous or nonmagnetic state.
Japanese Unexamined Patent Application Publication No. 2008-287853 discloses a magnetic recording medium having a composite magnetic recording layer including a first magnetic recording layer, a second magnetic layer, and a third magnetic recording layer. The first and second magnetic recording layers have a granular structure interposing a coupling layer in between. The third magnetic recording layer has an axis of easy magnetization in the direction perpendicular to the surface of the nonmagnetic substrate.
Japanese Unexamined Patent Application Publication No. 2009-157983 discloses a method of manufacturing a magnetic recording medium having a magnetic layer with a granular structure and a magnetic layer with a nongranular structure provided on the granular magnetic layer, the method forming a magnetic recording layer pattern with a magnetic separation structure, wherein the magnetic recording layer surface is slightly eliminated after removing a resist and a carbon mask layer.
Most of the magnetic recording layers used in perpendicular magnetic recording media employ a granular structure, in which magnetic particles are surrounded by oxides separating and isolating the magnetic particles. A magnetic recording layer entirely composed of the granular structure material, however, exhibits a large magnitude of a uniaxial magnetic anisotropy constant Ku and simultaneously a large value of magnetic inversion field, the latter being in proportion to the Ku value, resulting in rather degraded ease of recording.
Consequently, in order to improve performance of a magnetic recording medium as a whole, perpendicular magnetic recording media have nowadays become common in which the top layer of the magnetic recording layer does not have a granular structure and have a structure provided with a metal layer of an appropriate thickness, the metal layer consisting of a nongranular structure and being composed of continuous magnetic particles (metallic magnetic particles). Such a structure can achieve a good S/N ratio and an overwrite characteristic.
Most of the DTMs known in the art employ a procedure in which a machining step for creating protrusions and recesses is conducted after depositing a continuous film of a perpendicular magnetic recording layer. In the method, however, the protruding parts of the top layer of the magnetic recording layer (the metal layer) are exposed to various etching gases in the machining step for forming protrusions and recesses, resulting in damages of oxidation and corrosion beginning at the edge of the protruding parts.
As a consequence, the metal layer formed with an appropriate thickness cannot exhibit an expected performance, and the protruding parts of the magnetic recording layer greatly degrade in their performance of a magnetic recording medium. When the magnetic recording layer is covered with a carbon protective layer in a damaged state remaining corroded parts under the carbon protective layer and fabricated into a product of DTM, the product may be adversely affected in long term reliability due to elemental elution (or corrosion) in the future.
Japanese Unexamined Patent Application Publication No. 2009-076146 discloses a method of manufacturing a magnetic recording medium in which the corroded parts are eliminated by irradiating the whole surface of the magnetic recording layer with inert gas to stabilize the magnetic recording layer and suppressing migration of magnetic alloy even under a high temperature and high humidity environment.
This method, however, has a problem that an etching effect of the inert gas decreases a volume fraction occupied by the top magnetic layer resulting in a large magnitude of a coercivity Hc of the magnetic recording layer as a whole and a large value of an inversion magnetic field, which deteriorates ease of recording.
The inventor of the present invention has made extensive experimental studies and found that, in a magnetic recording medium having a continuous film to fabricate a DTM, a perpendicular magnetic recording medium with a high density three layer structure is rather possible to hold high density recording performance and suited to a DTM.
The inventor of the present invention has further found that a magnetic recording layer having a granular structure containing SiO2 filled between magnetic particles employed in ordinary perpendicular magnetic recording medium significantly exhibits a modification effect on magnetic properties by exposing it to reactive gas of halogen containing mainly fluorine and a magnetic separation effect in a DTM pattern.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.